Recently, photonic crystals have been drawing attention as an optical functional material. A photonic crystal is characterized in that its cyclic structure forms a band structure with respect to the energy of light or electromagnetic waves, thereby creating an energy region (called the photonic bandgap) that forbids the propagation of light or electromagnetic waves. It should be noted that this specification uses the term “light” or “optical” as inclusive of electromagnetic waves.
Introduction of an appropriate defect into the photonic crystal will create an energy level (called the defect level) within the photonic bandgap. This allows only a specific wavelength of light having an energy corresponding to the defect level to exist there within the wavelength (or frequency) range corresponding to the energy levels included in the photonic bandgap. Forming a linear defect will provide a waveguide, and forming a point-like defect in the crystal will provide an optical resonator. The resonance wavelength, i.e. the wavelength of light that resonates at the point-like defect, depends on the shape and refractive index of the defect.
Using such resonators and waveguides, research has been conducted to manufacture various types of optical devices. For example, the resonator can be located in proximity to the waveguide to create an optical multiplexer/demultiplexer capable of functioning as the following two devices: an optical demultiplexer for extracting a ray of light whose wavelength equals the resonance wavelength of the resonator from rays of light having different wavelengths and propagating through the waveguide, and for emitting the extracted light through the resonator to the outside; and an optical multiplexer for trapping a ray of light having the resonance wavelength of the resonator from the outside, and for introducing the trapped light through the resonator into the waveguide. Such an optical multiplexer/demultiplexer can be used, for example, in the field of optical communications for wavelength division multiplexing communication in which rays of light having different wavelengths are propagated through a single waveguide, with each ray of light carrying a different signal.
Photonic crystals can be created from both two-dimensional and three-dimensional crystals, of which two-dimensional crystals are advantageous in that they are relatively easy to manufacture. For example, Patent Document 1 discloses a two-dimensional photonic crystal and an optical multiplexer/demultiplexer, each of which includes a two-dimensional photonic crystal consisting of a plate (or slab) with a high refractive index and including a cyclic array of a material whose refractive index is lower than that of the material of the plate, where a waveguide is formed by linearly eliminating the cyclic array and a point-like defect (or a resonator) that disorders the cyclic array is formed in proximity to the waveguide. The cyclic array of the low refractive index material is formed by cyclically creating holes in the slab.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2001-272555 (paragraphs 0019-0032, 0055 and 0056; FIGS. 1, 22 and 23)
For a resonator utilizing the above-described two-dimensional photonic crystal, it is essential to make its Q-value as high as possible. Q-value is an index for the performance of a resonator, and a higher Q-value means a smaller amount of light leaking from the resonator to the outside. An increase in the Q-value of a resonator also means an improvement in the accuracy of an optical multiplexer/demultiplexer using the resonator as well as an improvement in the performance of the resonator itself. Specifically, in an optical multiplexer/demultiplexer, a larger Q-value of the resonator yields a higher wavelength resolution and thereby decreases the possibility that the noise components of light whose wavelengths differ from the resonance wavelength are multiplexed or demultiplexed, so that the multiplexing/demultiplexing accuracy improves.
In general, however, Q-values of two-dimensional crystals are lower than those of three-dimensional crystals because the light-confining effect of two-dimensional crystals is weak in the direction perpendicular to the slab face. In the optical multiplexer/demultiplexer disclosed in Patent Document 1, the Q-value of the resonator is approximately 500, and the full width at the half maximum of the spectrum of the light multiplexed or demultiplexed through the resonator is about 3 nm. These values are insufficient for high-density wavelength division multiplexing optical communication; the desired values are 0.8 nm or smaller in wavelength resolution and 2000 or larger in Q-value. Thus, it is desired to realize a two-dimensional photonic crystal resonator having a still higher Q-value.